BP's don't really "bask". They aren't fond of bright light or just hanging out in the open.
If you want to try it and see how it goes anyway, having your heat source on a reliable thermostat is even more important than usual because it can burn them a lot faster than ambient temperature or substrate temperature. Pretty much any mostly smooth stone that isn't really brittle and porous should work. You will want to clean it really well to make sure it doesn't have any chemicals or bacteria on it. If you don't care about natural aesthetics, a piece of tile would work great after sealing the bottom with a thin layer of 100% silicone caulk and letting it cure away from the snake for at least 48 hrs.